Google has quietly made a background update to Google Assistant that allows you to link your HBO NOW account to it. That means that you can stream content from the service to your Chromecast devices simply by asking Assistant to do so via your phone or Google Home.
HBO NOW is the stand alone streaming service from the premium movie service. It is $14.99 per month and is for cable cutters who do not have a cable or satellite subscription. For those that do have HBO as part of their cable package, the app to stream content on your Android device is HBO GO but that cannot be connected to Google Assistant.